In firefox i don't need to press a key to insert a coin or to start a game, in the top left corner of the browser is a little selector with Insert coin, Player 1 and Player 2 in it that can be clicked on.
It doesn't matter either way though i guess.
Also for some more info about the controls and stuff, read here-http://ascii.textfiles.com/archives/4423

Obviously it's still in it's early stages and there are some issues, notably with sound, but it should get better over time.
Personally i wouldn't play emulated games like this as i would prefer more control by using a proper emulator with roms.
But i'm sure web based emulation has it's purposes and could be handy at times.